Two of the most important sources of background information are:
Specialized encyclopedias:
Have articles that are on specific topics within a particular area (A general encyclopedia would have an article on Buddhism, a religion encyclopedia would have an article on a specific Buddhist ritual)
The articles are written by experts in the field
The articles provide bibliographies that list other sources on the same topic
The articles typically contain cross-references to other related topics
Provide an introduction to specialized vocabulary
Specialized dictionaries:
Provide definitions for specialized vocabulary
Often provide lengthy explanations associated with the vocabulary, more like a normal encyclopedia article